You didn''t mention the thermostat. Have you removed and test it? And what about calcification in hoses, etc? Is the water flow at the exhaust flow normal or diminished?

Also, just a thought, but are you absolutely sure the rubber impeller is firmly bonded to the bronze inner core?

I''m thinking that you''ve seen the impeller go around with the pump''s cover plate off, but the drag on the end plate is slowing or stopping impeller rotation in actual use. This was a real problem for many GM series Yanmars a few years back.

Otherwise, it''s hard to tell. What brand diesel? Have you cleaned the heat exchanger or just eyeballed one or both ends?